Restoring the default IPv6 route is not needed in CI and
fails when there are multiple defaults, but it is useful
in local devstacks where multiple default routes are not
typical.
Add a variable in settings and use it to make this behavior
conditional and set it to False for the lvm job.
Closes-bug: #1836788
Change-Id: Id73de8100509ec5935641f5f35f93f482d108bcd